1. Field of the Invention
The present invention is related to the field of recovering the hemicellulosic fraction, which is originating from lignocellulosic biomass.
The invention relates more particularly to a method for purifying pentoses, which are sugars forming the hemicellulose, while permitting the acid catalyst to be recycled, the latter being used for hydrolyzing the lignocellulosic biomass and for extracting the cellulose therefrom.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Traditionally, cellulose is used in the manufacture of materials, for example paper, by the paper industries.
Cellulose can also be converted to bioethanol by the so-called «2nd generation» bioethanol sector.
This sector permits to obtain biofuels from advantageously non-food plant components. In other words, there is no competition between this sector and a use of plants for food, as can be the case with bioethanol produced for example from rape or beet.
In particular, the starting plant components that can be used for obtaining 2nd generation biofuels are, for example, wood, green residues, cereal straw, fodder, forest residues, miscanthus, sugar cane bagasse, etc., the latter corresponding to the fibrous residue of sugar cane once the juice has been extracted therefrom. All these plant components can represent lignocellulosic biomass.
Cellulose is the major component of this lignocellulosic biomass, which can include up to 50% cellulose.
The extraction of cellulose from lignocellulosic biomass is generally carried out by implementing physicochemical techniques. In particular, a step of acid hydrolysis of the lignocellulosic biomass can be performed, which permits to recover a cellulose pulp. An enzymatic hydrolysis of the cellulose is then performed, in order to obtain glucose molecules, and the latter is then transformed into ethanol by fermentation, using yeasts.
In addition to cellulose, lignocellulosic biomass also includes hemicellulose and lignin.
Lignin consists of a macromolecule with a complex structure and high molecular weight.
The chemical structure of lignin is varying. The lignins are more particularly monolignol polymers, and there are at least three different ones: coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The fraction of each of the monomers in the lignin varies considerably and depends on various factors, such as the plant line, the species, the organ, or also the tissue.
As regards the hemicellulose, of which the lignocellulosic biomass is comprised for about 30 mass %, it is the 2nd major component of the pectocellulosic plant wall after cellulose. The hemicellulose plays a bridging role in this wall, between the cellulose fibers and other components of said wall.
The hemicellulose consists of a branched polysaccharide including different types of oses, unlike cellulose, which is exclusively formed of glucose molecules, and more particularly of D-anhydroglucopyranose units connected to each other by glycosidic bonds β (1→4).
As regards the ose monomers, which form the hemicellulose, they may be glucose, xylose, mannose, galactose, rhamnose or also arabinose. The most widely represented monomer within the hemicellulose is xylose.
The hemicellulose is a fibrous molecule, insoluble in water. Mostly comprised of pentoses, it is very slightly fermentable and it is therefore difficult to synthesize biofuels originating from hemicellulose.
Therefore, only the cellulose extracted from lignocellulosic biomass is readily convertible to ethanol.
However, techniques should be provided that permit to recover the hemicellulosic fraction from the lignocellulosic biomass, i.e. the fraction including the hemicellulose, and more particularly the oses, namely the pentoses, which the hemicellulose is partially comprised of.
Indeed, the pentoses, and more specifically the xylose, are of interest at the level of several industrial applications, such as the production of intermediate chemicals for xylitol, surfactants or resins.
The methods for purifying pentoses that are currently implemented at industrial level have changed little since the late 1980s. These methods have namely the disadvantage of being highly water- and chemical reagents-consuming, and they are in addition very polluting.
More precisely, once the acid hydrolysis of the lignocellulosic biomass has been performed in order to recover the cellulose, the reference method used so far to permit a purification of the sugars contained in the acid hydrolysate consists first of all in neutralizing the latter.
The neutralization of the acid hydrolysate is traditionally performed by adding thereto a mineral base such as lime or soda. This neutralization will cause the precipitation of the organic macromolecules, such as the proteins or also the lignin.
The precipitated organic macromolecules, as well as the suspended matters, are then removed by sedimentation or centrifugation. The hydrolysate is then demineralized by chromatography, as described in the patent documents U.S. Pat. Nos. 5,084,104, 6,239,271 and WO 2010/046532, by ion exchange, as described in U.S. Pat. No. 4,075,406, FR 2 655 661 and WO 2004/108739.
Finally, as described in U.S. Pat. No. 5,084,104 and WO 2010/046532, the hydrolysate obtained can be concentrated in order to obtain high-purity xylose crystals.
It can also be considered that the so treated hydrolysate is used for producing xylitol, surfactants, or resins.
However, the major disadvantage of these methods lies in that they are extremely polluting because they lead to rejecting about 90% of the acids used for the hydrolysis of the biomass.
Techniques developed more recently and described in the patent documents WO 2008/096971 and US 2012/0211366 have permitted to show the advantage of the electrodialysis for permitting the demineralization of the hydrolysate, after full or partial neutralization of the latter.
More specifically, patent document US 2012/0211366 relates to a method for producing xylose from a hydrolysate using the electrodialysis.
In this method, after a step of hydrolyzing the plant biomass, the pH of the hydrolysate being obtained, initially of about 0.8 to 1.2, must be adjusted, by adding sodium hydroxide, in order to reach a pH between 1.5 and 2.5. As a result, the acid catalyst loses most of its catalytic power and is not likely to be fully recovered and recycled.
In addition, here the addition of sodium hydroxide results, on the one hand, into an increase of the quantity of salts to be separated during the electrodialysis step and, on the other hand, into a precipitation of the soluble impurities at an acid pH. A clarification step is then necessary in order to eliminate the insoluble suspended matters, using a microfilter.
Anyway, in the methods known from the state of the art, the acid catalyst used in the hydrolysis of the biomass is little or not recycled. Indeed, its neutralization, or the adjustment of the pH to a certain value, requires a certain amount of a base. Therefore, the amount of salts to be separated during the demineralization step is considerably increased.
As a result, the existing methods require, for their implementation, a very large amount of water, energy and chemical reagents. Therefore, large quantities of waste water must be treated afterwards.
The patent document WO 2008/096971 has permitted to demonstrate the interest of using the electrodialysis technique in a method for recovering pentoses.
More particularly, in this document is described a method for producing xylitol from a hydrolysate including namely xylose and arabinose, the starting biomass consisting of tropical fruits.
In the method in question, after a conventional step of acid hydrolysis of the biomass, ions, namely sulfate ions and calcium ions, are precipitated by increasing the pH of the hydrolysate, which then changes from a pH 1 or 2 to a pH that can go up to 7.
However, even after this precipitation step, the hydrolysate is likely to still containing non-precipitated ions. The latter, although they are present in the hydrolysate at a low concentration, may result into the formation of a deposition during the subsequent step of concentration of the products of interest, ultimately resulting into a reduction in yield in the production of xylitol.
The implementation of an electrodialysis technique permits to eliminate the non-precipitated salts while limiting the use of ion-exchange resins, which inevitably requires steps of regenerating said resins, which are expensive in time and in chemical reagents.
However, in the method described herein, like in the other methods provided in the prior art, a step of raising the pH is necessary to remove the macromolecules and certain dissolved ions. Therefore, a large portion of the acid added at the time of the hydrolysis of the biomass is neutralized, which results into the loss of most of its catalytic power.
Moreover, the anionic or cationic membranes used in the electrodialysis technique are very sensitive to clogging and contamination by organic molecules. Therefore, the lifetime and the effectiveness of said membranes are likely to be reduced.
The macromolecules can also precipitate in the electrodialysis module, or stack, when the pH is increased.
Therefore, because of the high cost of the electrodialysis membranes, it is necessary to efficiently remove the components of the hydrolysate likely to precipitate, in order to be capable of implementing the electrodialysis technique at industrial scale.
It has already been mentioned that the macromolecules, in particular the lignin and the proteins, are precipitated by raising the pH of the hydrolysate. The latter, being initially of about 1 at the time of the acid hydrolysis, must be adjusted to a value between 2 and 7, for example by adding lime or a solution of soda to the hydrolysate, which inevitably results into the loss of most of the catalytic power of the acid.